Conventionally known is an epoxy resin powder coating composition prepared by mixing carboxylic anhydride as a hardener with an epoxy resin (e.g., Japanese Patent Application (OPI) Nos. 243168/85 and 89271/86). The term "OPI" as used herein means a published unexamined Japanese patent application. The aforesaid epoxy resin powder coating composition gives film having such excellent thermal resistance and mechanical strength that it has been used as an insulating coating compositions for electric and electronic parts. The coating composition, however, is highly hygroscopic and is apt to absorb moisture from the air. This impairs storage stability and lowers the hardening speed.